There exist many fields wherein accurate measurement of a time duration is critical to the success of a system analysis. In many instances, the time interval of measurement is determined by transmitting a relatively short pulse of energy (having a wide bandwidth) and precisely measuring the arrival time of the received return pulse. Typically, however, the return pulse is not identical to the transmitted pulse and in many instances the return pulse can be severely affected by the media through which the pulse travels. Typical examples of the use of a measured time interval are the radar and sonar environments wherein the time interval measures distance from the source to an object, for example, an airplane or the sea bottom. Another example of the use of time interval measurement is flow detection and measurement using ultrasonic signal energy, such as that described in Lynnworth, U.S. Pat. No. 3,575,050, issued April 13, 1971, wherein a short pulse of ultrasonic energy is transmitted through a moving fluid in an upstream and a downstream direction. The time intervals of upstream and downstream travel provide measurement data useful in determining fluid flow.
In many instances, the detection of the arrival time of an ultrasonic pulse passing through a fluid is affected significantly by factors such as turbulent flow conditions, high flow velocities, and changes in fluid temperature, pressure, and composition. Thus, an ultrasonic energy pulse propagating through a fluid can be subject to different and rapidly varying amounts of attenuation. However, in an ultrasonic measuring system, wherein it is the arrival time of the ultrasonic pulse which is relevant to the fluid property being measured, the detection process is often a difficult one due to amplitude variations of the received pulse. Typically, according to earlier apparatus, automatic gain control (AGC) circuitry has been employed for electronically reducing the amplitude fluctuations to improve both the reliability and the accuracy of the detection process.
Due to the pulsed nature of the received signals, wherein there are relatively long periods wherein no signal is present, a gated fast attack, slow decay type of automatic gain control, directed to "tracking" the signal envelope, has been traditionally employed. This type of automatic gain control can rapidly and accurately track an increasing signal due to its fast attack time; however, because the decay time is usually much slower than the rate at which the ultrasonic pulses are received, rapidly decreasing signals cannot be tracked.
Further, in many instances, the interval measuring apparatus will be employed in connection with measuring the arrival time along several different transmission paths. The amplitudes of the signals on the various paths will often differ considerably due to the differences in transducers employed, path geometry, and flow variables. It is also important in many instances to interrogate the paths quite rapidly. The variation in the received signal amplitude along the different paths, along different zones in a given path, or in opposite directions over a given path, however, can result in detection errors if the automatic gain control signal does not "follow" the separate paths accurately. Traditionally, therefore, when multiple paths have been employed, either a separate automatic gain control receiver has been employed in connection with each path or a single automatic gain control receiver has been employed with an interrogation rate on each path which is sufficiently slow to allow the automatic gain control amplifier sufficient time to correct for the different received amplitudes along the respective paths. The first method clearly requires a plurality of receivers which is costly and the second method is limited to a very slow interrogation rate, typically less than the desired pulse repetition rate.
In addition to the use of automatic gain control, in the ultrasonic flow measurement application in particular, the received pulse typically appears as though it were transmitted through a narrow band filter. Thus, in time, the extent of the pulse increases; and therefore, when accurate time durations are required, it is often difficult to exactly measure, consistently, when the pulse is received. In those instances where the time of receipt remains substantially constant from pulse transmittal to pulse transmittal, relatively standard procedures are available for accurately determining the time when the pulse is received. Thus, for example, a typical approach is to measure the amplitude of the returning pulse; and, when that amplitude exceeds a fixed voltage threshold value, to set the time of receipt as the time of the next zero crossing of the pulse signal. This method is adequate in relatively noise free environments, or where the transit time is relatively constant from measurement to measurement, and, under those circumstances, produces an accurate "relative" time duration. In the ultrasonic flow measurement system, it is the difference in transit time of the upstream and downstream pulse signals which is most important and hence the arrival time, if determined in a consistent manner (even if the time measurement contains a constant error), is usually adequate for measuring the flow within the pipe.
In many flowmeters, however, there is significant noise on the received signal from, for example, interference within the pipe due to either turbulence or pipeline irregularities. In other instances, the transit time varies significantly due to time varying flows and turbulence of the flow. As a result, a typical zero crossing measurement based upon the amplitude threshold method described above, proves inadequate to the task of determining, with a high degree of accuracy, the pulse receive time for a narrow bandwidth pulse signal. In essence, the difficulty is determining the same zero crossing, for example the fifth, for each and every pulse signal received.
It is therefore an object of this invention to accurately measure the time of arrival of a narrow bandwidth pulse signal. Another object of the invention is accurately determining the arrival time of an ultrasonic pulse signal in a volumetric flow measuring environment. Further objects of the invention are a reliable, accurate, easily maintained intervalometer apparatus and method for accurately determining the arrival time of a pulse signal under conditions of varying flow rates and turbulence of the flow. Yet further objects of the invention are an intervalometer method and apparatus which is cost effective to build and easy to manufacture.